Digitizers and tablets can be incorporated as a coordinate input apparatus in processing units. For instance, the digitizer or tablet can be used alongside one or more display devices (e.g. CRT, LCD, or other display technology) in a touch enabled display system. Generally speaking, various systems for detecting an angle (direction) or a position of an object relative to the display area can be used, such as pressure sensitive resistance membrane systems, capacitance systems, electromagnetic induction systems, and the like. As another example, optical systems capable of detecting the angle or the position of the object can be used. More particularly, touch screen input devices include resistive, surface capacitive, surface acoustic wave (SAW), infrared (IR), Frustrated Total Internal Reflection (FTIR), Projected capacitive, optical and bending wave. Often, the foregoing touch screen devices (aside from some optical and infrared technologies) require use of a touch enabled transparent cover layer that adds height to the display assembly.
Certain optical and infrared systems rely on detection of light traveling in optical paths that lie in one or more detection planes above the touched surface. For example, optical imaging for touch screens can use a combination of line-scan or area image cameras, digital signal processing, front or back illumination, and algorithms to determine a point or area of touch. Components used to emit and detect light in the detection plane(s) can be positioned along one or more edges of the touch screen area as part of a bezel surrounding the touch screen area.
Optical touch technology often uses line-scanning or area cameras orientated along one or more edges of the touch surface to image the bezel and track the movement of any object close to the surface of the touch screen by detecting the interruption of an infrared light source. For example, the light can be emitted across the surface of the touch screen by IR-LED emitters aligned along the optical axis of the camera to detect the intensity of light reflected by a retro-reflective border. Light can be projected from each of two optical units in a fan shape above the touch surface, with a linear photoreceptive sensor in each optical unit measuring the intensity of light detected by the optical units. If a finger, pen, or other object interrupts a portion of the light, the system can detect the direction of the shadow. Since the optical units are at known positions, the coordinates of the finger or pen can be calculated based on the triangulation principle.